1. Field of the Invention
This invention relates to compositions for use in papermaking. The invention also relates to methods of making and using paper products, for example, recording sheets, which include the composition.
2. Discussion of the Background
The demand in the marketplace is increasing for recording sheets, printing papers, writing papers, and the like which have superior printing and optical properties. To improve brightness and whiteness, for example, optical brighteners (OBAs) are being used in larger amounts. The OBAs are expensive, however, and their increased use contributes substantially to higher product costs.
To improve printing properties such as ink density and dry time, cationic metals have been used. Calcium chloride is currently used in ink jet recording media to enhance inkjet print density and dry time. See, for example, U.S. Patent Application Publication 2007/0087138, published on Apr. 19, 2007, which discloses a recording sheet with improved image dry time which contains water soluble divalent metal salts. Other metal salts have been used in inkjet recording media. U.S. Pat. No. 4,381,185 discloses paper stock which contains polyvalent metal cations. U.S. Pat. No. 4,554,181 discloses an ink jet recording sheet having a recording surface which includes a water soluble polyvalent metal salt. U.S. Pat. No. 6,162,328 discloses a paper sizing for ink jet printing substrate that includes various cationic metal salts. U.S. Pat. No. 6,207,258 discloses a surface treatment composition for an ink jet printing substrate which contains a divalent metal salt. U.S. Pat. No. 6,880,928 discloses an ink jet recording base paper having a coating which includes a polyvalent metal salt. It has been found, however, that many of these cationic additives decrease the brightness and whiteness. Calcium chloride, for example, undesirably quenches stilbene-based optical brighteners such as often used at the size press. Overcoming this decrease in brightness and whiteness imposes additional costs on the papermaking process.
Another disadvantage is that the use of certain cationic additives, such as calcium chloride can create runnability issues in paper machines; and calcium chloride affects the pH of size press formulations. Starches used at the size press require a narrow pH range to be effective: too high of a pH may result in the yellowing of the starch; too low of a pH may cause the starch to precipitate and/or gel. Calcium chloride can also interact with other chemicals such as those used in the wet end when the paper is broked or recycled.
Synergistic mixtures of complexing agents, such as the known chelant, diethylenetriaminepentakis(methyl)phosphonic acid (DTPA), and polyacrylic acid have been used to enhance brightness in chemical and mechanical pulps. See, for example, U.S. Pat. No. 7,351,764. Chelating agents have also been used to produce acid-stabilized calcium carbonate slurries. See, e.g., U.S. Pat. No. 7,033,428. U.S. Patent Application Publication 2007/0062653 discloses that the use of reducing agents in combination with certain chelants enhance the brightness of a paper product via increased thermal stability of the pulp and reduction of chromophoric structures in pulp. There, it is disclosed that chelants include compounds that are capable of chelating transitional metals that form colored products with pulp constituents and catalyze color-forming reactions in the bleached pulp or paper products.
There is thus a need for a recording sheet with improved optical properties yet which reduces the costs associated with OBAs.